Sadly, my french is even worse than my engrish
Err I guess it's because that one source just happens to have lower values? I'll try to give an analoge example:
I know people donate on average 50$ to charity, which gives them a tax benefit of 30% of that 50$; population is 500 000
In my sample, people of one region donate on average 37$ to charity, which gives them a tax benefit of 25% (benefit is unrelated to region, i'm just saying region instead of that one provider) on that 37$; sample size is around 1300. I can calculate mean, deviation, range (of donation size) etc from the sample size because I have all values
I want to know the range (more or less) of the population
I think I formulated it wrong, the data provider is just my one source, or region (see above) from whom I get 'data' aka sample.
Sorry for all confusion

I think I may be mixing up english terms.
I have 1 data provider out of X (unknown amount of providers), with this one provider I got 1500 "subjects" of the population. Is this "big enough"?
How exactly can I determine how it is unrepresentative? For example with the mean of the sample size and the mean of the actual population?

Writing a paper and my puny statistics knowledge can't handle all the info (which I don't have). Decided to make a thread rather than status update because more questions may follow in the next weeks.
Data I have:
A small sample size (represents roughly 1/400 of population)
The actual mean of the population
Question numéro uno:
Is it possible to calculate a range of values of the population? The problem is that the mean of the data that I have is quite far under the actual mean, and thus (I hope) the highest values as well.*
*part of my research is around the very extreme values, there are other not so extreme but still higher values that may cause the mean to be higher but wouldn't benefit my study as much